Molecular and electronic structures of cerium and cerium suboxide clusters Free

Bulk cerium oxides have electronic properties that are desirable for applications in electronics^1–3 and catalysis.^4–10 It has recently been observed that the more oxidation states a metal can access, the more diverse its applications can be,¹¹ and cerium oxides can famously switch between the +3 and +4 oxidation states with changes in O₂ partial pressure (a feature that makes it particularly useful catalytic converters).^12–14 Recently, several high profile papers have propelled interest in cerium oxides as a support material for platinum because of the strong support/catalysts interactions that evidently enhance the activity toward the water-gas shift reaction.^6,15 Further enhancement is achieved with nanoscale cerium oxide support, which points to the importance of local interactions and defect sites in catalyst activity. Recent theoretical studies have highlighted the role of oxygen vacancies on ceria for coupling water to the Pt (and other) catalysts.^15–18 

Our research program has explored metal oxide clusters in a wide range of lower-than traditional oxidation states as models for oxygen-vacancy defect sites,^19–21 which are commonly implicated in catalyst activity.^22–24 Because of the local nature of defect sites and catalyst-substrate interactions, metal oxide cluster models offer a helpful alternative approach to surface studies.²⁵ Recently, we completed a study on reactions between water and small cerium oxide cluster anions that showed very distinct oxidation state-dependent product distributions.²⁶ Other groups have also used cluster models to study the properties of cerium oxide. For example, Akin et al. found patterns in the photodissociation of Ce_xO_y⁺ clusters with a strong distinction between clusters for which y = 2x − 1 and y > 2x − 1, indicating a change in how O-atoms bind to the cationic clusters at this particular change in stoichiometry. Wu et al. observed size-dependent rates of reactions between Ce_nO_2n+1⁻ and CO for CO₂ formation.²⁷ Cerium oxyhydroxide cluster anions studied by Aubriet et al. showed the propensity to form species in which the cerium atoms were fully oxidized (+4).²⁸ Hirabayashi et al. reported results of several reactivity studies on cerium suboxide cluster cations showing average Ce-oxidation state dependence on reactions leading to oxidation of CO and NO,²⁹ as well as cerium suboxide cluster cation reactions with O₂.³⁰ Mafuné and co-workers have also probed reactions between small Ce_xO_y⁺ suboxide³¹ and hyperoxide³² clusters with a range of small molecules.³³ 

Very few spectroscopic studies of cerium oxides have been reported beyond the CeO diatomic, which has been the subject of numerous spectroscopic^34–38 and ab initio^39–42 investigations, and the insightful ligand field theory (LFT) treatment applied by Field.⁴³ Asmis and co-workers used infrared predissociation spectroscopy combined with DFT calculations to determine the structures of a range of small Ce_xO_y⁺ suboxide clusters, and showed that small clusters bore structural similarities to the bulk.⁴⁴ They also have similarly determined the structures of small cerium/vanadium oxide cluster cations.⁴⁵ Willson and Andrews with co-workers measured the matrix-isolated IR spectra of a series of early lanthanide metal containing molecules generated by laser ablation and reactions with small molecules.^46–49 We have obtained the anion photoelectron (PE) spectra of CeO⁻ and Ce(OH)₂⁻,⁵⁰ and Chi et al. measured low-energy photoelectron images of CeO⁻ along with several other LnO⁻ diatomics (Ln = La through Nd).⁵¹ The anion PE spectroscopic technique offers the advantage of mass specificity and has been applied toward a number of lanthanide metal-based species.^52–56 

Here, we present the photoelectron (PE) spectra of a series of Ce_xO_y⁻ (x = 2–5; y < 2x) clusters in very low oxidation states. The spectra generally exhibit transitions to close-lying neutral electronic states anticipated to arise in electronically complex species. Consider the cerium atom: The ¹G₄ ground state of the atom arises from the 4f 5d 6s² orbital occupancy (bulk Ce has 4f 5d² 6s), and ca. 60 electronic states are found within 1 eV of the ground state, a consequence of energetically close-lying 4f, 5d, and 6s orbitals.⁵⁷ In analyzing the PE spectra with qualitative portraits of the electronic structures of the anion and neutrals species provided from DFT calculations, we map out the evolution of the electronic structure as a function of oxidation, which gives insight into the initial stages of Ce oxidation, a highly exothermic process (Ce powder is pyrophoric). As seen in a previous study on LaO_n⁻ for n = 1–5,⁵² the electron affinities of the Ce_xO_y clusters do not increase smoothly with oxidation state, as is commonly observed for main group and transition metals.⁵⁸ This report will be followed by a companion study on the electronic structure of the products formed from reactions between these clusters and water.⁵⁹ 

Ce_xO_y⁻ PE spectra were collected using an apparatus described in detail previously.⁶⁰ Ce_xO_y cluster anions were generated using a laser ablation/pulsed molecular beam valve source.⁶¹ Approximately 3-5 mJ/pulse of the second harmonic (532 nm) output of a Nd:YAG laser, operating at a repetition rate of 30 Hz, was used to ablate the surface of a metal target composed of compressed Ce (Alfa Aesear) powder. The resulting plasma was entrained by a pulse of ultra-high purity helium buffer gas (40 psi backing pressure) introduced from a solenoid-type molecular beam valve, and swept through a 2.5 cm-long, 0.3-cm diameter channel into a vacuum chamber. After collimation of the gas mixture by a 3-mm skimmer, the anions were accelerated on axis into a 1.2-m beam-modulated time-of-flight mass spectrometer.

Before colliding with a dual microchannel plate ion detector, the anions were selectively photodetached using the second (532.1 nm, 2.330 eV) or third (354.7 nm, 3.495 eV) harmonic output of a second Nd:YAG laser at the intersection of the ion drift tube and a perpendicular 1-m field-free drift tube. The drift times of the small fraction of photoelectrons that traveled the length of the drift tube and collided with a second dual microchannel detector assembly were recorded on a digitizing oscilloscope.

Spectra were accumulated between 80 000 and 200 000 laser shots, and were measured with laser polarizations parallel (θ = 0°) and perpendicular (θ = 90°) to the electron drift tube with intensities I₀ and I₉₀, respectively, in order to determine the asymmetry parameter, β(E),

which can be related to the symmetry of the molecular orbital associated with electron detachment.

For calibration purposes, the drift times were converted to electron kinetic energy (e⁻KE) by identifying common transitions observed in spectra of various Ce-based anions with similar electron affinities (EA), collected using both photon energies, and setting the difference in the electron kinetic energies (e⁻KE) to the fundamental energy (1.1650 ± 0.0001 eV), the difference between the energies of the second and third harmonics, using the relationship,

where m_e is the electron mass and t_3ν is the drift time of electrons associated with a selected transition observed in the spectrum obtained using 3.49 eV photon energy that can readily be correlated with a transition in the spectrum obtained with 2.33 eV, appearing at t_2ν. The equation is solved for ℓ, and plotted as a function of t_o. The intersection between this line and other lines generated from several sets of transitions observed in 3.49 eV and 2.33 eV gives a unique ℓ and t_o, the calibration parameters necessary to compute the e⁻KE values from electron drift times. The e⁻KE values are related to the anion and neutral states via (2) taking into account internal energies for the anion and neutral species,

The data presented show electron counts plotted as a function of e⁻BE,

The e⁻BE values reflect the energy difference between the final neutral state and the initial anion state, and are independent of the photon energy used. Laboratory to center-of-mass frame corrections were made to the e⁻KE (and e⁻BE) values.

Molecular and electronic structures of Ce_xO_y⁻ anions and neutrals were explored using density functional theory. Calculations were performed using the unrestricted B3LYP hybrid method within the Gaussian 09 program suite.⁶² This method was chosen because of helpful results obtained for CeO, Ce(OH)₂, PrO, EuH, EuO, and EuOH anion and neutral species in previous studies.^50,55,56 To incorporate relativistic effects on the Ce metal atom, the Stuttgart RSC ANO/ECP basis set with 28 core electrons and contraction of (14s 13p 10d 8f 3g)/[6s 6p 5d 4f 3g] type, developed by Cao and Dolg, was employed,⁶³ with the Dunning-style correlation consistent basis set, aug-cc-pVTZ for the oxygen atoms. Geometry optimization and frequency calculations were performed for all the anion and neutral species in multiple spin states. Numerous initial guess structures in a large range of possible spin states were attempted for all anions and neutrals. The results presented below focus on the lowest energy structures found for the anions (all structural isomers which were found to be energetically competitive were considered) and the neutral species accessible from those anions. The supplementary material includes figures presenting all structures that converged for the anions and neutrals, and their relative energies. Generally, we found structures with more Ce—O—Ce bridge bonds to be more stable than those with Ce=O terminal bonds. Adiabatic detachment energies (ADEs) of 1 − e⁻ transitions between anion and neutral states with comparable structures were calculated from the difference between the zero point-corrected energies of the optimized anion and neutral species structures. Photodetachment spectroscopic parameters were gleaned from the optimized anion and neutral structures, vibrational frequencies, and normal coordinates and used to generate simulated spectra using home-written LabView codes for a more quantitative comparison between the experimental and computational results.

A characteristic mass spectrum of the Ce_xO_y⁻ anion distribution generated by ablation of Ce metal is shown in Figure 1. For each value of x, there is a fairly narrow range of suboxide species, with the Ce_xO⁻(y = 1) cluster being most abundant for all values of x. In all cases, with the exception of x = 2, the metallic cluster (y = 0) is observed, though Ce₅⁻ was not observed in sufficient quantities to measure the spectrum. PE spectra and computational results are summarized in Figures 2–14. A general observation on the computational results is that the structures of most species show minor distortion from high symmetry structures, and the orbitals are often highly spin polarized, making electronic state symmetry assignments difficult. Additionally, for all the Ce_xO_y⁻/Ce_xO_y clusters, there are 7x nearly degenerate 4f-based molecular orbitals, and different occupancies will result in different overall electronic term symmetry. The more salient features are the qualitative orbital descriptions and spin states. In cases where the overall symmetry is ambiguous because of distortion and polarization, the states are simply labeled ^2S+1A.

PE spectra of Ce₂O⁻ and Ce₂O₂⁻ are shown in Figures 2(a) and 2(b), respectively, with transition energies summarized in Table I. The PE spectra were obtained using 3.495 eV (blue traces) and 2.330 eV (green traces) photon energies. The dark blue and green traces are spectra collected with the laser polarization parallel to the photoelectron drift tube (θ = 0°), and the light green and blue traces are spectra collected with perpendicular polarization (θ = 90°).

The spectrum of Ce₂O⁻ exhibits two overlapping transitions near an e⁻BE value of 1.1 eV, labeled X and A, respectively. The origin of band X is 1.06(2) eV, which can be taken as the EA of Ce₂O. The vertical detachment energy (VDE) of band X is 1.09(2) eV. The origin of band A is difficult to identify because of overlap with band X; the VDE is 1.16(2) eV. The values in parentheses represent the uncertainty in the last digit. At first glance, band A could conceivably be a member of a 560 cm⁻¹ vibrational progression originating from peak X. However, based on the profile of the spectrum and on the computational results (vide infra), we tentatively assign these features to distinct electronic transitions. Bands X and A have asymmetry parameters of 0.5(1) and 0.8(1), respectively.

The spectrum of Ce₂O₂⁻ [Fig. 2(b)] shows two very distinct bands labeled X and A at 0.83(2) and 1.05(2) eV, respectively. Both bands exhibit partially resolved but irregular structure. Bands X and A have asymmetry parameters of 1.2(1) and 0.9(1), respectively. We note here that the EA of Ce₂O₂, which can be taken to be 0.83(2) eV, is 0.23 eV lower than the EA of Ce₂O.

Both spectra exhibit low intensity features at approximately 1.2 eV higher in binding energy relative to band X, indicated by asterisks (*). Similar features have been observed in the PE spectra of CeO⁻ and PrO⁻, and were tentatively assigned to shake-up transitions.^50,55 For both CeO⁻ and PrO⁻, the HOMO is a doubly occupied 6s-like σ orbital. The energetically viable shake-up transitions involve detachment of one of the 6s electrons accompanied by promotion of the other 6s electron into a 5d-like orbital. CeO and PrO neutral excited states that arise from the 4fⁿ5d superconfigurations approximately 1 eV higher in energy than the 4fⁿ6s ground state superconfiguration. Based on the calculations presented in the next several paragraphs, the transitions observed in the Ce₂O⁻ and Ce₂O₂⁻ also involve detachment from molecular orbitals that have large contributions from Ce 6s orbitals, so we similarly tentatively assign the features labelled (*) to shake-up transitions.

Results of calculations on Ce₂O anions and neutrals suggest that Ce—O—Ce bridge bonding is a favored structural motif, as summarized in Figure 3. Table II includes results on several of the anion and neutral states to illustrate how close-lying the various states and the predicted transition energies are. Atomic coordinates for these and additional states along with more details from the computational outputs are included in the supplementary material. C_2v structures were clearly favored for Ce₂O⁻, while a nearly linear but low-symmetry Ce—O—Ce (∠_Ce—O—Ce = 171°) structure was calculated to be the lowest energy neutral structure, with C_2v structures found 0.15 eV higher in energy.

Again, we do not presume to get an exact description of the electronic structure of these clusters from the computational results, but in qualitative terms, the electronic structure of the [Ce_aOCe_b]⁻ anions can be described as ...$[4fa4fb]σ6s2[Cea−Cebbondingorbital](5d)(σ6s*)2$⁠. In this description, we have foregone the symmetry of the orbitals in favor of a description of the bonding character of the orbitals. The 4f-based molecular orbitals have very little overlap, and their occupancy can be described as a single electron occupying a single 4f orbital on each of the two Ce atoms. These 4f orbitals lie energetically between the Ce—O bonding orbitals and the highest-lying Ce 5d and 6s-based molecular orbitals. The Ce—O bonding orbitals are dominated by O 2p orbitals, with some contribution from Ce 5d orbitals, consistent with predominantly ionic bonding. The term symbols included in Fig. 3 take into account the symmetry of the 4f orbitals, with the caveat that shifting electrons from the orbitals depicted here into any of the other 4f orbitals is very unlikely to change the overall term energy significantly, since from a bonding standpoint, these orbitals are core-like.

The exact identity of the singly occupied Ce 5d–Ce 5d based orbital also does not affect the overall bonding picture in Ce₂O⁻. Based on the calculations, occupation of either a σ _5d or π _5d orbital, both of which are shown in Fig. 3, gives nearly isoenergetic states, with the σ _5d occupancy favored by 0.06 eV. States with doublet spin multiplicity formed when the spin of the electron in the 5d-5d orbital is antiparallel to the two unpaired electrons in the 4f orbitals are predicted to be 0.1–0.2 eV higher in energy than the quartet states. The 5d-based orbitals are also nearly isoenergetic with, though slightly lower in energy than, the $σ6s*$ orbital, so numerous neutral states predicted to be within 0.3 eV of the calculated ground neutral state could in principle be accessed via detachment of numerous close-lying anion states.

As a starting point for assigning the spectrum, we generated simulations based on computational results on one of the nearly degenerate lowest-energy quartet states of the anion, $[4fa4fb]σ6s2(π5d)(σ6s*)2$ to the quintet (“blue” electron) and triplet states (“red” electron) with $[4fa4fb]σ6s2(π5d)(σ6s*)$ orbital occupancy, shown in Figure 4. Simulations based on results for the analogous quintet and triplet states with the singly occupied σ_5d orbital rather than π_5d were nearly identical. Vibrational frequencies and general structural parameters are summarized in Table II; a more extensive list of spectroscopic parameters (normal coordinate displacements, transition origins, peak widths, and temperature) is included in the supplementary material. The ⁵B₁ − ⁴A₂ transition (blue trace) features fairly extended progressions in the 95-100 cm⁻¹ bend mode (2 cm⁻¹ anharmonicity was used in calculating peak positions), which is unresolved in our spectrum, and very little excitation of the 460–470 cm⁻¹ symmetric stretch. The symmetric stretch progression is slightly more extended in the ³B₁ − ⁴A₂ simulation. Detachment of an electron from the $σ6s*$ orbital affects the Ce—Ce equilibrium bond distance, as summarized in Table II, but is fairly non-bonding with respect to Ce—O.

The transition from either of the quartet anion states to the nearly linear ...$[4fa4fb]σ6s2(σ6s*)2$ state (detachment of the “green” electron, Fig. 3) is calculated to originate at lower electron binding energy, but based on the substantial structural change associated with this transition as well as the smaller photodetachment cross sections for 5d versus 6s orbital detachment,⁶⁴ we tentatively assign the very low-intensity signal observed from ca. 0.7 eV up to the origin of band X to this transition. Note that there is also broad signal to higher binding energy in PE spectrum. Because the σ_6s and $σ6s*$ orbitals are separated by less than 1 eV in the calculations, it is possible that detachment from the σ_6s orbital may contribute to some of the signals in the e⁻BE > 1.3 eV range.

Figure 5 shows the lowest energy structure calculated for Ce₂O₂⁻, which is nearly identical to the lowest energy structure found in calculations on Ce₂O₂, both with D_2h symmetry. Depictions of the highest singly and doubly occupied orbitals are also shown. In broad terms, the electronic structure of Ce₂O₂ can be related to the Ce₂O⁻ molecule as having Ce—O bonding orbitals that are dominated by O 2p orbitals (three of the six are shown in Fig. 5), one singly occupied 4f orbital on each of the two Ce atoms, and as expected, two fewer electrons in the higher lying Ce-local orbitals, which can be described as σ_6s and $σ6s*$⁠, with an overall occupancy of ...$[4fa4fb](σ6s)2(σ6s*)$⁠. The anion is predicted to have a ⁴A_g ground state, with the ²A_g state in which the $σ6s*$ electron spin is antiparallel to the two 4f electrons 0.02 eV higher in energy. This very small quartet-doublet splitting is comparable to an analogous splitting in the CeO diatomic molecule: the X₁2(j_f = 5/2, j_s = − 1/2) − X₁3(j_f = 5/2, j_s = 1/2) splitting, 0.01 eV, reflects how decoupled electrons in the 4f and 6s orbitals are. The predicted ³B_1u neutral ground state has the ...[4f_a 4f_b](σ_6s)² orbital occupancy, and the ⁵A_g state with ...$[4fa4fb](σ6s)(σ6s*)$ occupancy is predicted to be 0.08 eV higher in energy. Calculations on the ³A_g state with the same occupancy but antiparallel spins of the electrons in the (σ_6s) and $(σ6s*)$ orbitals did not converge. A summary of the computational results is included in Table II. Again, we note that shifting the Ce 4f-local electrons into alternative nearly degenerate orbitals will result in nearly identical, close-lying states with different overall electronic species.

Based on the overall orbital occupancy of the Ce₂O₂⁻ anion, we assign band X to transitions associated with detaching an electron from the singly occupied $(σ6s*)$ orbital, and band A to detachment from the (σ_6s) orbital. Simulations of the ³B_1u − ⁴A_g (blue trace) ³B_1u − ²A_g (blue dotted trace) and ⁵A_g − ⁴A_g (red trace) transitions based on calculated spectroscopic parameters are shown in Figure 6. Vibrational frequencies of the totally symmetric modes and general structural parameters are summarized in Table II; a more extensive list of spectroscopic parameters (normal coordinate displacements, transition origins, peak widths, and temperature) is included in the supplementary material. The energies of the origins were shifted by less than 0.1 eV in order to match the spectrum, and the order of the transition energies was preserved. Note that the electronic configurations of the two anion and two neutral states involved in the simulations point to additional transitions, ³A_g − ²A_g and ¹A_g − ²A_g, slightly higher in energy than the ⁵A_g − ⁴A_g transition, and band A in the experimental spectrum may appear broader than the simulated spectrum because of these overlapping transitions. The near vertical appearance of simulated spectra indicates that the (σ_6s) and $(σ6s*)$ orbitals are non-bonding. However, the highest a_g stretch frequency determined from calculations, and indicated on Fig. 6, does appear to be active in band A in the experimental spectrum, though as noted above, it is also possible that several electronic transitions are contributing to band A.

The PE spectra of Ce₃O_y⁻ (y = 0, 1) are shown in Figure 7, and the spectra of Ce₃O_y⁻ (y = 2–4) are shown in Figure 8. As before, the blue traces are spectra obtained with 3.495 eV photon energy, and the green traces are the spectra obtained using 2.330 eV photon energy; darker traces indicate laser polarization parallel (θ = 0°) to the direction of electron drift, and lighter traces indicate orthogonal (θ = 90°) polarization. The EA’s of this series of clusters again do not change monotonically with oxidation state: In order of increasing oxidation for Ce₃⁻ to Ce₃O₄⁻, they are 0.67(1), 0.89(5), 0.95(5), 1.00(1), and 0.792(5) eV. The ADE and VDE values are summarized in Table III.

The spectra of Ce₃⁻ and Ce₃O⁻ [Figs. 7(a) and 7(b)] exhibit a dominant band labeled X, with notably higher intensity at θ = 0° laser polarization. Qualitatively, the PE spectra of Ce₃⁻ and Ce₃O⁻ are similar: Band X is fairly narrow, ca. 0.09 eV full width at half maximum (FWHM), and there are two additional broad electronic bands labeled A and B at higher binding energy. Band A in both has a more isotropic angular distribution, with the asymmetry parameter, β(E), approaching zero in the spectrum of Ce₃O⁻ obtained with 3.49 eV photon energy. Band B in both spectra has a lower intensity and suffers from low signal to noise and accompanying continuum signal, but the polarization dependences for both Ce₃⁻ and Ce₃O⁻ are similar to band X. Relative energies of all bands in the Ce₃O_y⁻ spectra are included in Table III.

The PE spectra of Ce₃O_y⁻(y = 2–4) [Figs. 8(a)–8(c)] are also dominated by their respective lowest e⁻BE transition, labeled band X, and all three have varying undulations of lower-intensity signal to higher e⁻BE. As with the spectra of Ce₃⁻ and Ce₃O⁻, there is continuum signal in the Ce₃O₂⁻ and Ce₃O₃⁻ spectra from band X up to e⁻BE = hv, and it is more intense in the Ce₃O₂⁻ spectrum. Bands X in the Ce₃O₂⁻ and Ce₃O₃⁻ spectra are relatively broad, 0.25 eV FWHM. The Ce₃O₂⁻ spectrum exhibits a progression of shoulders spaced by 500 cm⁻¹, which is typical of a Ce—O—Ce stretch frequency, and can be modeled with a Franck-Condon simulation (not shown) assuming a normal coordinate displacement of ΔQ = 0.75 Å amu^1/2. Computational results described below do not point to a clear structural determination, however. The PE spectrum of Ce₃O₃⁻ has partially resolved vibrational structure that can be reconciled with computational results described below, and a low intensity band labeled with an asterisk (*) at approximately 0.7 eV higher in energy. Band X in the PE spectrum of Ce₃O₄⁻ is nearly vertical and exhibits a low-intensity and fairly narrow excited state transition labeled with an asterisk (*) at e⁻BE = 1.25(3) eV [T₀ = 0.46(3) eV]. Only the spectra obtained with θ = 0° laser polarization are shown; spectra collected with θ = 90°had low signal intensity, giving β(E) ≈ 1.5(2).

Pictorial summaries of the results of calculations on the molecular and electronic structures for the lowest energy anion and neutral trimetallic cerium oxide clusters are shown in Figures 9–11, with a summary of the various electronic state relative energies and several structural and vibrational details found in Table IV. A more extensive listing of computational results is found in the supplementary material.

The anions and neutrals calculated for most species are compact, and all O-atoms are in bridging positions. Results for Ce₃/Ce₃⁻ and Ce₃O/Ce₃O⁻ are shown in Fig. 9. Again, gleaning a qualitative picture of electronic structures of the anions and neutrals, the Ce atoms in all anions and neutrals can be described as having a singly occupied 4f orbital. The Ce₃/Ce₃⁻ structures are either D_3h or Jahn-Teller distorted C_2v structures. The HOMO in the C_2v structure is a 6s-6s antibonding orbital with respect to two of the three Ce centers. This orbital is singly occupied in the lowest energy neutral structure and doubly occupied in the anion. Taken together with the a₁ (or a₁′ for the D_3h structure) orbital that can be described as predominantly the in-phase combination of all three 6s orbitals, the HOMO can be interpreted as one of the Jahn-Teller distorted components of the degenerate 6s-based e′ orbitals. Interleaved energetically with the largely 6s-based orbitals are 5d-based bonding orbitals, two of which were predicted to be singly occupied. Neutral Ce₃ therefore can be described as three Ce-centers with 4f 5d² 6s occupancy, which is bulk-like.⁶⁵ 

Upon bonding with a single O-atom, the Ce centers in Ce₃O⁻ are still electron rich. Two nearly isoenergetic C_3v and C_s structures in which the oxygen atom is either bound to all three cerium atoms (pyramid structure) or bridging two of the Ce atoms (kite structure) were found computationally (the computational results generally have these structures distorted slightly from C_3v and C_s). From a qualitative standpoint, the O-atom bonds with the Ce₃ portion again through 2p–5d overlap, though there is more contribution from 2p orbitals, which is consistent with a largely ionic bond. However, comparing the orbital occupancies of Ce₃⁻/Ce₃ and Ce₃O⁻/Ce₃O, electron density into the O²⁻ is drawn from the antibonding 6s-based orbital and one of the 5d-based orbitals. The orbitals and occupancies depicted in Fig. 9 correspond to the ⁴E Ce₃O⁻ pyramid structure, which was predicted to be marginally lower than the kite structure; on the neutral surface, the kite structure is slightly lower in energy. The calculations predict that the single electron in the Ce₃O⁻ HOMO is antiferromagnetically (AF) coupled to the 4f electrons, though spin states in which there is some AF coupling between the 4f electrons are also calculated to lie in a very small window of energy.

A pictorial summary of results of calculations on Ce₃O₂⁻ is shown in Fig. 10. Two structures, house and distorted kite, were found to be very close in energy for both the anion and neutral. The orbitals and occupancies shown are for the distorted kite structure. Comparing the Ce-local orbitals in Ce₃O₂⁻/Ce₃O₂ to Ce-local orbitals in Ce₃O⁻/Ce₃O in Fig. 9, the additional O-atom again bonds primarily through overlap with the 5d-based MO’s, and because of largely ionic character, the resulting bonding orbitals are predominantly 2p-like. Therefore, the 6s-based MO occupancies of Ce₃O⁻ and Ce₃O₂⁻ appear similar, with the difference being the occupancy of the 5d-based MO’s.

For both Ce₃O⁻ and Ce₃O₂⁻, we cannot determine unambiguously which structures are contributing to the spectra based on the computational results, though from an electronic structure and transition energy standpoint, the different structural isomers are similar.

Calculations on Ce₃O₃⁻ and Ce₃O₄⁻ are summarized pictorially in Fig. 11. A C_s book structure was the only low-energy structure found for Ce₃O₃⁻. Neutral Ce₃O₃ favors an open ring structure, with the book isomer lying 0.2 eV higher in energy. The Ce centers in Ce₃O₃ appear to have the same 4f6s orbital occupancy as CeO, with the extra charge in the anion localized in a 6s-like orbital (the quartet and sextet spin states for the anion have the same occupancy, with the unpaired electrons in the highest 6s-based MO’s being either parallel or antiparallel to the electrons in the 6s-based a″ orbital).

As summarized in Table IV, the ADE values calculated for all of these species are in reasonable agreement with the spectra, and it appears that sequential oxidation of Ce₃ does not affect the electron affinity significantly. The direct conclusion of this observation is that the anion and neutral cerium clusters are stabilized equally by oxidation. The various close-lying structural isomers found for Ce₃O⁻ and Ce₃O₂⁻ also have nearly identical ADE values. Spectral simulations generated using parameters from the unique structures found for Ce₃O₃⁻ and Ce₃O₄⁻ are shown in Figures 12(a) and 12(b), respectively. For Ce₃O₃⁻, we assumed that the unique book structure found for the anion only accesses the neutral book structure (the higher-lying neutral isomer), and the simulation shows only the sextet to quintet transition. The simulated spectrum is better resolved than the experimental spectrum, but we note again that there are close lying electronic states with different spin multiplicities, all of which can be contributing to the spectrum. The origin of the simulation was shifted up by 0.08 eV relative to the calculated ADE to match the experimental profile. Overall, the profile of the simulated and experimental spectra is in reasonable agreement.

The calculation-based simulation of the Ce₃O₄⁻ spectrum is in near-perfect agreement with the experimental spectrum, though the origin was shifted up by 0.23 eV relative to the calculated ADE value to match the observed spectrum. This disparity is within the ±0.3 eV error in ADE expected in our calculations,⁶⁶ though it is larger than other results presented here. The spin densities for the anion and neutral states are included in Fig. 12(b). The orbital occupancy shown for the quartet spin state of Ce₃O₄⁻ [Fig. 11] exhibits broken symmetry, and the two singly occupied (antiparallel spin) orbitals may be a doubly occupied orbital which is expressed with significant spin polarization in the calculations. In the spin-pure neutral quintet state, the singly occupied HOMO is the symmetric combination of the three 6s orbitals.

Qualitatively, we can compare the orbital occupancies and relative energies with the decrease in the number of distinct excited state transitions observed in the spectra as Ce₃O_y⁻ oxidation is increased. First, the O-local bonding orbitals are energetically inaccessible with the photon energies used in this experiment, generally lying >5 eV lower in energy than the HOMO’s. The 4f-based MOs are also energetically near the upper limit of the photon energy and are core-like non-bonding orbitals with very low photodetachment cross section.⁶⁴ Therefore, the detachment signal we observe is associated with 5d- and 6s-based orbitals, which are energetically interleaved for Ce₃O_y⁻, y = 0–2. The 5d-based orbitals are vacant, for y = 3, 4, and are the anti-bonding counterparts of the predominantly 2p-local bonding orbitals.

The grouping of valence orbitals predicted for Ce₃⁻ and Ce₃O⁻ (Fig. 9) suggests that band X in both spectra can be assigned to detachment of an electron from their respective 6s-6s antibonding HOMO, band A in both spectra may be overlapping lower-intensity transitions associated with detachment from the three close-lying 5d-based bonding orbitals found ca. 0.5 eV lower in energy than the HOMO’s, and band B in both spectra may be associated with detachment of the in-phase 6s bonding orbitals predicted to be ca. 1 eV lower in energy than the HOMO’s. The difference in the relative energies of the 5d-based bonding orbitals for Ce₃⁻ and Ce₃O⁻ relative to their HOMO’s, −0.7 eV and −0.3 eV, respectively, agrees with band A observed at lower e⁻BE in the PE spectrum of Ce₃O⁻. Furthermore, the disparity between the β parameters of band A versus band X in both spectra might be expected for transitions associated with orbitals having significant differences in node structure.

The general decrease in higher e⁻BE signal with increasing oxidation (Ce₃O₂⁻, Ce₃O₃⁻) is consistent with a depletion of electrons occupying the close-lying 5d and 6s-based MOs as well a bonding environment that further breaks the degeneracy of 5d and 6s-based molecular orbitals. With multiple electrons occupying numerous nearly degenerate molecular orbitals, the likelihood of short-lived electronic excitation of the anion followed by autodetachment to multiple close-lying neutral states is more likely. Additionally, shake-up (two electron) transitions may be occurring, as suggested for the smaller x ≤ y lanthanide oxide species. As oxidation increases, fewer electrons occupy any close-lying Ce-local orbitals that remain. The Ce₃O₄⁻ PE spectrum, with a single dominant transition and clean baseline to higher binding energies, is easily reconciled with the calculations assuming that the spin-polarized orbitals shown in Fig. 11 correspond to a real picture of a doubly occupied 6s-based orbital. There are no other occupied orbitals other than the 4f core-like and 2p-local bonding orbitals. The spin of the remaining electron in the diffuse HOMO skimming the perimeter of the cluster is not coupled to the spin of the 4f-local electrons; transitions to either the higher or lower spin state are too close in energy to resolve: A comparable 4f 6sJ_a = 2 − J_a = 3 splitting in CeO is 82 cm⁻¹,³⁴ and this splitting could not be resolved in the PE spectrum of CeO⁻ measured with the same apparatus.⁵⁰ 

The PE spectra of Ce₄O_y⁻ (y = 0–2) shown in Figure 13 exhibit intense, broad, and possibly overlapping transitions dominating the spectra at binding energies similar to those observed in spectra of smaller clusters. The EA’s of Ce₄, Ce₄O, and Ce₄O₂ are 0.97(5) eV, 0.94(2) eV, and 1.01(4) eV, respectively. As with the most reduced members of the Ce₃O_y⁻ series, continuum signal is observed along with several distinct excited state transitions. Band X in the Ce₄⁻ spectrum is broad relative to Ce₃⁻, and the X-A energy interval, 0.8 eV, is larger than the X-A interval in Ce₃⁻. The spectra of Ce₄O⁻ and Ce₄O₂⁻ both exhibit a low-lying excited state transition, labeled A, overlapping with, and more intense than band X. Peak positions and asymmetry parameters are summarized in Table V.

The PE spectra of Ce₅O_y⁻ (y = 1–2) shown in Figure 14 are also dominated by two comparably intense bands, X and A, and the continuum signal in both spectra is more intense than in the smaller clusters. The adiabatic electron affinities of Ce₅O and Ce₅O₂ are 1.09(5) and 0.96(3) eV, respectively, comparable to all other clusters included in this study. Peak positions and asymmetry parameters are included in Table V.

Given the electronic structures predicted for the Ce₂O_y⁻ and Ce₃O_y⁻ cluster anions and neutrals, the positions and intensities of the Ce₄O_y⁻ and Ce₅O_y⁻ are consistent with detachment of electrons from molecular orbitals dominated by Ce 6s orbital contributions. The continuum signal will be considered more carefully in the discussion section.

The evolution of the electronic structure of these small cerium oxide clusters in the initial stages of oxidation provides a point of comparison to the bulk. Starting with the metallic species, the electronic orbital configuration of the Ce atomic ¹G₄ ground state is 4f 5d 6s²,⁵⁷ which is not favorable for bonding⁶⁷ The bulk band structure of metallic Ce decomposed into atomic orbitals has the 4f 5d² 6s occupancy, which is comparable to the orbital occupancy of Ce₃, predicted by the DFT results. Likewise, Ce₂, the spectrum of which could not be obtained, has analogous electronic structure based on high level ab initio calculations reported by Nokolaev,⁶⁷ and Cao and Dolg.⁶⁸ 

Figure 15 shows a schematic of how the predicted electronic structure of the Ce₃O_y⁻ series evolves with y, and how it compares the Ce₂O₃ bulk band structure based on DFT calculations reported by others.^69,70 All Ce centers in the Ce₃O_y⁻ clusters included in this study and bulk Ce₂O₃ have singly occupied 4f orbitals, which are situated at zero energy in the plot (E − E_f for bulk Ce₂O₃). The schematic shows that while the 6s- and 5d-based orbitals in Ce₃ are interleaved energetically, electrons are depleted from the Ce-local 5d orbitals upon bonding with the O-atoms; the molecular orbitals with greatest Ce 5d contribution become unoccupied antibonding orbitals, which ultimately correlate to the conduction band of Ce₂O₃ and CeO₂. The point at which x = y, each Ce atom has lost two electrons from the 5d orbitals, leaving each Ce center nominally with the 4f6s superconfiguration. While the companion to this report will show how subsequent oxidation shifts towards more distinctly ionic bonding, up to x = y, the Ce—O bonding can be described as largely ionic bonding, with some covalent character, as evidenced by the small contribution of Ce 5d orbitals in the 2p dominated bonding orbitals.

The structures calculated for Ce₃O₄⁻ and Ce₃O₄ are the same as the structure determined in IR predissociation measurements on Ce₃O₄⁺.⁴⁴ This is a case in which a high-symmetry structure is unaffected by charge state, unlike several examples presented above, and the numerous cases we have found previously in transition metal suboxide anion and neutral structures.¹⁹ In accord with Asmis and co-workers,⁴⁴ we note that the structures of Ce₃O₄⁻, Ce₃O₄, and Ce₃O₄⁺ are evocative of the CeO₂ (111) surface. The (111) surface of CeO₂ was calculated to be the lowest energy surface,⁷¹ and though no detailed description of the Ce 4f orbital occupancy at the (111) surface, either O- or Ce-terminated in oxidizing or reducing environments has been published to our knowledge, the apparent stability of this common Ce₃O₄⁺/Ce₃O₄/Ce₃O₄⁻ cluster may point to 4f orbital occupancy as a source of this stability. Oxygen termination of the reduced (111) surface has been proposed as a precursor to the phase transition between CeO₂ and Ce₂O₃, for which there is the obvious change in 4f orbital occupancy.⁷¹ 

The PE spectra of the range of suboxide clusters shown in this report have several peculiarities worth noting. First, the less oxidized the cluster, the more pronounced the continuum signal is above the adiabatic detachment energy. In addition to the occupied and partially occupied 5d-bonding orbitals, there are additionally low-lying 5d-based unoccupied orbitals (each Ce center has five 5d-orbitals, and only a maximum of 2 e⁻ per Ce atom are occupying them) so the anions and neutrals necessarily have numerous excited states associated with promoting the 5d-orbital electrons into other easily accessible 5d-orbitals. The continuum signal may conceivably be thermionic emission or autodetachment that follows electronic excitation. The fact that the continuum signal varies with photon energy and generally increases with cluster size (and therefore an increase in close-lying states) is consistent with the either picture, since both would be enhanced with increasing densities of electronic states.

Second, there are no significant variations in the electron affinities of the series of Ce_xO_y⁻(y = 0–4) clusters included in this study. They all fall within several tenths of an eV from 1 eV, and based on the computational results, the ground state transitions are all associated with detachment of an electron from a delocalized molecular orbital that primarily has contributions from Ce 6s orbitals. This includes previously published results on Ce⁻,⁷² CeO⁻, Ce(OH)₂⁻,⁵⁰ and unpublished result on larger Ce_xO_yH_z⁻ clusters generated from Ce_xO_y⁻ + H₂O reactions.⁵⁹ Regardless of whether the 6s-based molecular orbitals are bonding or antibonding in character, the anisotropy parameters measured for the ground state transitions are all positive, with β(E) ∼ 0.5–1.5. This finding gives an interesting picture of how the electronic structure of a finite cerium suboxide cluster deviates from the bulk stoichiometric oxides, in which the 6s band lies high in energy above the 4f-5d conduction band: The cluster HOMO orbitals which are dominated by Ce-6s atomic orbitals are fairly non-bonding and delocalized over the whole cluster, and the detachment energy associated with these orbitals seems unaffected by size and extent of oxidation in the range of clusters we have studied. By extension, considering the propensity of metal oxides to accumulate negative charge under a range of applications, small cerium suboxide particles may have the metallic property of electron delocalization over the surface.

The anion photoelectron (PE) spectra of Ce₂O_y⁻(y = 1, 2), Ce₃O_y⁻(y = 0–4), Ce₄O_y⁻(y = 0–2), and Ce₅O_y⁻(y = 1, 2) were reported and analyzed with a qualitative insight into the cluster electronic structures from density functional theory calculations. The PE spectra all exhibit an intense electronic transition to the neutral ground state, all falling in the range of 0.7–1.1 eV electron binding energy, with polarization dependence consistent with detachment from Ce 6s-based molecular orbitals. There is no monotonic increase in electron affinity with increasing oxidation, which is direct evidence that the anions and neutrals are equally stabilized with stepwise oxidation.

A picture of how electronic structure evolves with oxidation state was formulated by comparing the experimental anion PE spectra and the computational results. The Ce₂⁻ metallic dimer was not observed experimentally, but the trends inferred for sequential oxidation of Ce₂O⁻ to Ce₂O₂⁻ were borne out in the more complete progression of oxidation determined for Ce₃O_y⁻. The electronic structure of the smallest metallic cluster observed in this study, Ce₃, is consistent with the bulk structure in terms of atomic orbital occupancy (4f 5d² 6s). Initial cerium cluster oxidation involves largely ionic bond formation via Ce 5d and O 2p orbital overlap (i.e., O 2p localized), with Ce—O—Ce bridge bonding favored over Ce=O terminal bond formation. With subsequent oxidation, the Ce 5d-based molecular orbitals are depleted of electrons, with highest occupied orbitals described as diffuse Ce 6s based molecular orbitals. In the y ≤ (x + 1) range of oxidation states, each Ce center has a singly occupied non-bonding 4f orbital. The PE spectrum of Ce₃O₄⁻ is unique in that it exhibits a single nearly vertical transition. The highly symmetric structure predicted computationally is the same structure determined from Ce₃O₄⁺ IR predissociation spectra,⁴⁴ which resembles the Ce-terminated (111) surface of CeO₂.

Spectra of clusters with x ≥ 3 exhibit considerable continuum signal above the ground state transition; the intensity of the continuum signal decreases with increasing oxidation. This feature was attributed to a plethora of partially occupied, nearly degenerate Ce 5d and 6s-based molecular orbitals, which give rise to the possibility of numerous close-lying quasibound anion states that undergo autodetachment to a range of neutral states, as well as two-electron transitions. Upon oxidation, the nearly degenerate orbitals separate energetically into O 2p-local bonding and Ce 5d-local anti-bonding orbitals while the O-atoms accumulate electrons from the previously partially occupied 5d and 6s-based orbitals, decreasing the density of low-lying electronic states in both the anion and neutral.

See supplementary material for details on spectroscopic parameters used in the Ce_xO_y⁻ PE spectral simulations, comparison of Ce₂O two disparate 5d-based orbital configurations, diagrams summarizing various cluster structural isomers and their relative energies, and a complete list of Cartesian coordinates for all structures that converged in the calculations.

This work was supported by the National Science Foundation Grant No. CHE-1265991.